The RPR test or qualitative rapid plasma reagin test is used to diagnose syphilis, a sexually transmitted disease (STD). The syphilis test screens for antibodies in the blood produced as a result of a Treponema pallidum (T. pallidum) infection, the bacterium that causes syphilis.

Syphilis is usually spread by sexual contact, such as through direct contact with a syphilis sore (chancre). These sores are firm, raised and painless, and typically appear on the genitals or in the throat. Syphilis is easily treated if caught early. If left untreated, the disease may be passed to sexual partners and cause severe health problems. A pregnant woman can also pass the disease to her fetus, with serious and potentially fatal consequences for the infant.

Multiple types of tests detect syphilis. Antibody tests such as the RPR test are the most common. This test is a nontreponemal antibody test, meaning it detects the presence of antibodies, but they may not be specific to the T. pallidum bacterium. Such antibodies are produced as a result of syphilis, as well as other conditions. Therefore, a positive result should be followed by a treponemal antibody test, such as the FTA-ABS test, to confirm the diagnosis. These tests detect antibodies that specifically target T. pallidum; other conditions are unlikely to produce a positive result. A patient that has contracted syphilis, even if successfully treated, will always carry treponemal antibodies. Nontreponemal antibodies, on the other hand, tend to disappear in about three years following successful treatment. Therefore, both types of tests will be required to either confirm the diagnosis (if the nontreponemal test is taken first) or to distinguish between an active infection/reinfection or past infection (if the treponemal test is taken first).

A negative RPR test result does not mean the patient is free of syphilis, particularly if the exposure was recent. It can take up to several weeks for antibodies to reach detection levels in the blood. As a result, repeat RPR testing may be required at a later date.

*A quantitative RPR test may be used to monitor the course of syphilis treatment.